The present invention relates generally to implantable cardiac leads, and more particularly to endocardial leads having small diameter lead bodies.
Over the past few years, there has been a substantial effort to reduce the diameter of endocardial pacing, cardioversion and defibrillation leads. Reduction in lead body diameter facilitates placement of multiple leads through a single blood vessel and also minimizes interference between the lead body and the tricuspid valve, in the case of leads implanted in the right ventricle. In addition, a reduction in lead body size has generally been accompanied by a reduction in the lead body""s flexural stiffness or bending moment. This reduction in lead body flexural stiffness has generally been seen as an advantage, in that it reduces the possibility of perforation of heart tissue by the distal tip of the lead.
Some of the considerations underlying the quest for leads of ever-decreasing size and flexural stiffness are discussed in U.S. Pat. No. 5,246,014 issued to Williams et al, which discloses a lead having a diameter in the 1-2 French range. Small diameter defibrillation leads are disclosed in U.S. Pat. No. 5,871,530, also issued to Williams et al.
While small diameter pacing leads with highly flexible lead bodies do reduce the possibility of perforation and do facilitate the passage of multiple leads through the same blood vessel, e.g., the cephalic or subclavian vein, it has been determined that very small diameter and very flexible leads tend to cause substantial damage within the heart, due to their tendency to be rapidly moved within the heart chamber in a xe2x80x9cwhip-likexe2x80x9d fashion. This damage may result in the formation of substantially more coliform vegetations (fibrous nodules) on the heart wall and valve leaflets than would be caused by stiffer leads.
The leads of the present invention retain the advantages of a small diameter lead body while avoiding damage due to whip-like movement of the lead body within the heart chamber. The present invention accomplishes this result by means of a lead body employing an insulation material that has a substantially higher Young""s modulus (tensile stiffness) than would normally be employed in the context of a permanently implantable cardiac pacing lead.
The properties of the inventive lead structure may be considered using the principal that the flexural stiffness, or bending moment, of a fiber or tube varies with the tensile modulus, and the diameter to the 4th power. That is, for a tube, the bending moment, or flexural stiffness is determined as:
(Young""s modulus)xc3x97(OD4xe2x88x92ID4)/64
wherein OD and ID represent the Outer and Inner Diameters, respectively, of the tube, and Young""s modulus is a measure of tensile stiffness of the material employed to manufacture the tube.
According to the current invention, a very stiff and even brittle material such as glass or carbon can be used to make flexible fabrics by using them in the form of very fine fibers. Such materials may provide a lead body which has a bending moment approximating that of prior art large diameter leads, while retaining the desired small diameter lead body. Because the bending moment of the leads of the current invention remains relatively unchanged, the possibility of perforation of the heart tissue by the distal tip of the passively fixed (tined) lead body using conventional electrode assemblies is not increased. The pressure that can be applied to the endocardium by the distal tip, as measured in pounds per square inch, is therefore similar to that of presently implanted larger leads. Alternatively, the distal tip may be a corkscrew which, when inserted within the cardiac tissue, prevents perforation. The electrode typically has a surface area of about 1.5 to 6 mm2, allowing the lead to retain desirable electrical characteristics for sensing of cardiac depolarizations and delivery of cardiac pacing pulses.
In one embodiment, the outer diameter of the current lead is smaller than 3.0 French, and may be as small as 1.0 French. The lead body insulation is formed of Pellethane 2363-55D, 75D, or an even stiffer polymer, such as Genymere polyimide (Virginia Power Nuclear Services Company). The material selection may depend upon the tubing diameters required. In the context of a larger diameter pacing lead, including bipolar coaxial leads wherein two tubings are required, such materials would generally provide leads far too rigid for permanent implant, risking perforation of the myocardium and possible tamponade which can be lethal. In the context of a lead having an outer diameter of 3 French or less, however, the minimal cross-sectional diameters of the insulation produces a lead body having overall bending moment characteristics similar to prior art, larger diameter leads employing silicone rubber or softer polyurethanes as insulative materials.
According to one aspect of the invention, the distal end of the lead body may be designed to present a cross-sectional area similar to that provided by prior art larger diameter leads of similar lead body bending moment. A similar surface area pacing electrode may be located in this tip. The pacing electrode may be, for example, a porous sintered electrode with the capability of eluting a glucocorticosteroid such as dexamethasone or beclamethasone, similar to those described in U.S. Pat. No. 5,282,844 issued to Stokes et al and incorporated herein by reference in its entirety. Alternatively, the pacing/sensing electrode might be arranged as a xe2x80x9cring tipxe2x80x9d electrode, extending around the external periphery of the distal portion of the lead body, resembling the electrode disclosed in U.S. Pat. No. 5,342,414 issued to Mehra, also incorporated herein by reference in its entirety. Other electrode configurations, including the use of multiple small electrodes disbursed across the distal tip surface of the lead body and/or active fixation electrodes may also be substituted.